Show dream as idle mode.

This changelist shows the currently configured dream over
keyguard as the idle mode. A timeout then transitions the
dream to doze.

This is a stopgap solution as the dream is rendered using
existing windowing behavior outside of SystemUI.

Test: manual
Bug: 194230499
Change-Id: I101b3d89e7bca63aec398c0a0edffaa6957f39d0